In recent years, applications of resin-based composite materials (hereinafter, simply referred to as “composite materials”) in which a matrix resin is reinforced by fiber to members of a wind turbine blade, an airplane, an automobile, a marine vessel, a railroad vehicle, and the like have been spreading rapidly due to a significant advantage offered by such composite materials in terms of weight reduction.
A known method for manufacturing a composite material is autoclave molding in which a plurality of sheets of laminated prepreg material is covered by a bag film and pressure-formed by vacuum suction, and further pressurized and thermally cured in an autoclave.
For example, Patent Document 1 describes autoclave molding which prevents wrinkles and undulations from occurring in a composite material by performing molding while applying tension to a unidirectional fiber prepreg. This molding method involves laminating a plurality of sheets of unidirectional fiber prepreg on top of a forming female mold, overlaying a forming upper template on top of the prepreg, holding an end of the unidirectional fiber prepreg with a tension plate fixed to the forming female mold and a holding plate which is placed on top of the tension plate and which presses the prepreg, overlaying a breathable fabric and a bag film on top of the forming upper template, vacuuming an inside of the bag film, and curing the prepreg by applying heat in a state where the prepreg is pressurized.
However, although autoclave molding is capable of producing a tough composite material because the composite material is baked in an autoclave while being pressurized, autoclave molding requires an autoclave which is a large-scale facility and is too expensive for mass production. In particular, with a composite material that is used in large structural members such as a wind turbine blade or a wing of an airplane, manufacturing high-quality products at high productivity becomes a major issue in addition to obvious requisites for material property such as tensile strength as well as fatigue strength and long-term durability.
In consideration thereof, a vacuum impregnation method (VaRTM: Vacuum assisted Resin Transfer Molding) which does not require a large-scale facility is attracting lots of attention. The vacuum impregnation method involves covering a fiber-reinforced base material placed on a forming die with a bag film, vacuuming an inside of the bag film, injecting liquid resin into the bag film, and curing the liquid resin.
For example, Patent Document 2 discloses a vacuum impregnation method in which a reusable and transparent silicone sheet is used as a bag film. With this method, since a thin-walled, transparent silicone sheet is used as the bag film, a weight of the bag film is reduced and, at the same time, the transparency makes a condition of a liquid resin flowing inside the bag film visible. Therefore, since the bag film can be handled with greater ease and an impregnation condition of the resin can be visually confirmed, workability during composite material production can be improved.    Patent Document 1: Japanese Patent Application Laid-open No. 1-110-296864    Patent Document 2: Japanese Patent Application Laid-open No. 2010-115837
However, with the evolution of end products such as wind turbines, airplanes, automobiles, marine vessels, and railroad vehicles, further improvements in strength are desired for composite materials that are used as members in these end products. Therefore, a conventional vacuum impregnation method such as that described in Patent Document 2 may no longer be able to provide composite material with sufficient strength.
For example, with a structural member such as a wind turbine blade, compressive strength is used in static strength design. Therefore, an improvement in compressive strength is desired for composite materials. In this regard, with a conventional vacuum impregnation method such as that described in Patent Document 2, since tension is not applied on fibers during molding, a straightness of the fibers is low and a composite material with high compressive strength cannot be obtained.
In addition, with a conventional vacuum impregnation method such as that described in Patent Document 2, since a force does not act on the fibers in a length direction of the fibers during molding, wrinkles are likely to occur in the fibers and may cause a decline in the strength of the composite material.